Linking coordinates between layers

Question

I have two point layers each with a single point in them. I want to draw a line joining the points to use to measure the distance apart.

I have found various plug-ins which allow this, but is there a way I can actually link the point coordinates to the line layer so that if I move one of the points the line will update automatically?

Answer 1

Install the plugin Points2One pluginand follow the dialog to create a line from your points. Save as geojson, this should provide you with lines between the two points, the part about moving the points coordinates and the layer updates automatically that would be possible, as far as I'm aware.

Answer 2

Just about your "the line will update automatically" part, a virtual layer may help.

In this example I have prepared two point layers

• my_From.shp layer with a field from_id
• my_To.shp layer with a field to_id

And the actual workflow to create a line (virtual) layer is:

1. Layer | Add Layer | Add Virtual Layer
2. Embedded layers - [Import] above two layers
3. In the Query field, give a query (as below).
4. Click on [OK], and a virtual layer with connected lines will be created.
5. NB. when you add a point in a layer, please make sure you have identical ids in from_id and to_id.

Query:

SELECT 
    my_From.from_id,
    my_To.to_id,
    make_line(my_From.geometry, my_To.geometry) geometry
FROM my_From CROSS JOIN my_To
WHERE my_From.from_id = my_To.to_id

[EDIT] To include a field line_length, please use a query below. Labeling with this field is also possible.

SELECT 
    my_From.from_id,
    my_To.to_id,
    make_line(my_From.geometry, my_To.geometry) geometry,
    st_distance(my_From.geometry, my_To.geometry) AS line_length
FROM my_From CROSS JOIN my_To
WHERE my_From.from_id = my_To.to_id

Lastly, when you add a new point please open the layer properties of the virtual layer and click on [Apply] to refresh the layer. (so it is semi-automatic..., not 100% automatic... sorry).